IRJET- Density-Aware Rate Adaptation for Vehicle Safety Communications in the Highway Environment

International Research Journal of Engineering and Technology (IRJET) e-ISSN: 2395-0056
Volume: 05 Issue: 03 | Mar-2018 www.irjet.net p-ISSN: 2395-0072
© 2018, IRJET | Impact Factor value: 6.171 | ISO 9001:2008 Certified Journal | Page 2154
DENSITY-AWARE RATE ADAPTATION FOR VEHICLE SAFETY
COMMUNICATIONS IN THE HIGHWAY ENVIRONMENT
P.Steffy Sherly1, G.Vanisha2, S.B.Sowmiya3, S.Senthamizhselvi4
1,2,3 B.E., Department of CSE, Panimalar Engineering College, Chennai, Tamil Nadu, India
4 Associate professor, Department of CSE, Panimalar Engineering College, Chennai, Tamil Nadu, India
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Abstract - The implementation of safety applications in
vehicular ad hoc network (VANET) depends on the
dissemination of safety related messages. A self-sortingMAC
protocol is proposed for high-densityscenarios.Theprotocol
allows vehicles to sort with others in a collision-tolerance
manner before data transmission. The vehicles establish a
logic queue by the self-sorting process, and the queue isable
to access the channel once the length reaches the set
threshold. Vehicles in the queue will access the channel by
time-division multiple access (TDMA) when the queue
occupies the channel. A queue will compete for accessingthe
channel on behalf of all the nodesin the queue,whichgreatly
alleviates the contention for access from all nodes. In
contrast with completely random access, the slot a queue
select to access the channel dependson the completion time
of the self-sorting process. In this case, the queue
accomplishing the self-sorting process first can avoid
collisions with other queues since they are still in the self-
sorting process. The performance oftheproposedprotocolis
evaluated compared with other typical MAC protocols in
VANET. The analysis and simulation results in highway and
city scenarios show that the proposed protocol can
significantly reduce packets loss and delay especially in
dense scenarios.
1. INTRODUCTION
VEHICULAR ad hoc network is a self-organizing network
aiming to improve the transportation safety and efficiency
via various safety applications over vehicle-to-vehicle(V2V)
and vehicle-to-infrastructure (V2I) communications. The
safety application is the primary motivation for deploying
vehicular communications, which depends on the
dissemination of status messages containing velocity,
position, etc. Vehicles broadcast their statusmessagesevery
100 ms for cooperative collision avoidance (CCA). The
broadcast messages for safety applications are used for
locating the vehicles in a collision threat or in blind spot and
sending warnings when a collision or a sudden brake is
detected.
The main challenge in VANET is the channel congestion in
MAC layer in high density scenarios.
There are large amounts of devices sending their status
messages in the transmission range, which will lead to
serious packet collisions and increase the time delay. A
suitable maximum delay requirement for the time-critical
safety applications is 100 ms. It has been specified that the
packet delivery ratio (PDR) should be not less than
90%.Simulationbased analysis show the impact of high
density on latency and PDR.
The congestion control issues have been investigated
extensively and various MAC protocolshave been proposed
to improve the performance of VANET. Since the safety
related message are mainly transmitted by broadcast, there
is no acknowledgement message for a successful reception.
Thus, the sender is unable to adjust the contention window
since it is unaware of any packet collisions. The authors
which present a scheme for dynamic adaptation of
transmission power and contention window based on the
estimated local vehicle density and collision rates. The
authors analyze the performance of carrier sense multiple
access(CSMA)based broadcast networksin VANET byusing
toolsfrom stochastic geometry. A theoretical analysismodel
is the performance of the distributed coordination function
(DCF) MAC protocol in IEEE 802.11p, which is based on
CSMA, and a retransmission algorithm is proposed to
improve the reliability of the system. However, theextended
analysis showsthat the retransmissionalgorithmonlyworks
well under low density circumstances, but not suitable for
high density cases since the extra packets of retransmission
triggered by a failure transmission will aggravate the
channel congestion.
A considerable number of cluster-based protocolshavebeen
proposed to improve the performance of VANET. A cluster-
based scheme ispresented in using the contention free MAC
within a cluster and the contention-based IEEE 802.11 MAC
among cluster-head vehicles. Each vehicle is equipped with
two transceivers operating on different channels. The
authors in present a distributed multichannel and mobility-
aware cluster-based MAC (DMMAC) protocol, which allows
vehiclesto send updated statusmessagesto the clusterhead
every 100 ms. The transmission range is decreased to make
a compromise to guarantee the communicationof all cluster
members, which will hinder the performance of safety
applications. It provide a taxonomy and comparative
performance review for the existing cluster based protocols
in VANET, and the main aspects of the clustering problem
are discussed. The majority of cluster-based algorithms
attempt to maintain the cluster topology through
complicated control processesin a long timespan,wherethe
performance can be affected by the mobility of vehicles.
1.1 SELF-SORTING PROTOCOL
The self-sorting protocol consists of three steps for vehicles
with packets to send: self-sorting, channel reservation and

data transmission. There is no tight and universal
synchronization, neither fixed structure of three phases in
the proposed protocol. The transition of the phases is all
trigged by particular situatione.g., once the length ofaqueue
reaches the set threshold in the self-sorting process, the
queue will start the channel reservation immediately, and if
the queue occupies the channel successfully, membersinthe
queue will transmit their data messages in the order of
joining the queue. There is no need to maintainthestructure,
since the queue will be automatically dismissed afterthelast
member finishes its data transmission.
1.2 Self-Sorting
Vehicles with a nonempty buffer aim to join in a self-sorting
process or start a self-sorting process. When detecting the
existence of a queue in the self-sorting process, the vehicle
will compete to join the queue. Otherwise, the vehicle will
start a self-sorting processby itself and become atemporary
head of queue (QH) with the probability.
1.3 Channel Reservation
Queues will start the channel reservation process
immediately when the length of the queue reaches the
threshold t. The channel reservation mechanismisproposed
to reduce the collisions caused by hidden terminals on such
conditions: If quested within the range of each other’s for
data transmission finish the self-sorting process at the same
time slot and start data transmission immediately, the data
packets from different queues will conflict in the overlap
area.
1.4.Data Transmission
The QH completing the channel reservationaftersendingthe
third reservation declaration tone will start the data
transmission. First, the QH will send the data packet which
consists of safety information and the sequence of the nodes
in the queue. It is used to inform the nodes in the queue
which miss the ACK message in the self-sorting process. The
nodes in the queue will broadcast their data packets
following the sequence from the head to the tail in TDMA.
Then the channel is released, and nodes with packet to send
will start a new self-sorting procedure and repeat the
process above. Compared with existing slotted MAC
protocols, only the data messagesfor safety applications are
transmitted in the data transmission phase, and there is no
extra control messages included in the packets, e.g., the
allocation information of each slot in ALOHA and TDMA-
based protocols aforementioned.
2. ANALYTICAL MODEL
2.1.System Model
We analyze the performance of the MAC protocol proposed
in terms of the mean delay and the PDR, and the superiority
in overhead is compared with the typical slotted protocols.
2.2.Probability of Successful Queuing
In this subsection, we construct a Markov chain to derivethe
probability of forming a queue successfully. In the self-
sorting process, a smaller transmission range Rlis applied,
and the transmission range of the declaration messageis2Rl
to avoid overlap of adjacent queues. If a node has packet to
send, it will become a temporary QH with the probability of
P1 by sending three short QH declaration messages in the
range [−2Rl, 2Rl].
2.3. Service Opportunity
In this subsection, we calculate the average service
opportunity that a node can acquire for transmission after
the self-sorting and the channel reservation process. If the
node is in a queue which has successfully formed a queue
with length of t, it has the opportunity to transmit its data
packets in the transmission process. Otherwise, it just hasto
compete for the service opportunity in following processes.
2.4.Implementation Overhead
We analyze the implementation overhead of the self-sorting
protocol proposed in this paper, and it is compared with the
typical slotted MAC protocols: TDMA-based and ALOHA
based protocols, since they are the most relevant to this
paper. For the fairness and the objectivity of the analysis,
principles are established as follows:
i. The packet sizes except for the implementation
overhead introduced by different protocols are
the same.
ii. We define a parameter, the implementation
efficiency (IE), to compare the overhead
performance of different protocols. The IE is
equal to the size of the payload of a packet
divided by the total bits to transmit the packet.
Table -1: ARCHITECTURAL DIAGRAM

3. PERFORMANCE EVALUATION
To evaluate the proposed protocol in terms of delay and
PDR, we conduct a series of analysis and simulations and
present the results in this section. The simulations arebased
on highway and city scenarios. The performance of the
proposed protocol is evaluated with different parameters
configurations.
The highway scenario is based on a one-direction highway
segment, with the velocity of vehicles ranges from 80-120
km/h, which is typical for highways. The city scenario
consist of a horizontal street, a vertical street and four
squares. The intersection of two streets is referred to as a
junction area. Vehicles move into the junction area will
choose possible direction with equal possibility. Vehicles
located at the junction area can communicate with vehicles
within transmission range on both streets. For a vehicle not
at the junction area, it can only communicate with vehicles
within transmission range on the same street due to the
existence of city blocks.
However, in ultra-dense scenarios, the performance of the
protocol proposed will degrade. It is because that the
collisions become seriousin the self-sorting processinultra-
dense scenarios. And after a queue successfully occupiesthe
channel, the number of nodes in the queue which have the
opportunity to transmit their data messages is constant
whether in low density or high density situations, which
means that the nodes will take more times to get a position
in a queue. Therefore, the performance of the proposed
protocol improves with the density first and then degrades.
In spite of the degradation of performance in extremely
dense scenarios, the self-sorting protocol shows the
superiority compared with other MAC protocols in VANET.
0.1 0.2 0.3 0.4 0.5 0.6
0.7
Vehicle Density (vehicles/m)
Compare the performance of the self-sorting protocol in
highway and city scenarios. It can be observed that the
performance of the proposed protocolinhighwayscenariois
better compared with the performance in city scenario at
same vehicles density. The reason is that, in city scenario,
vehicles not located at the junction area cannot receive the
channel occupation messages from the other streets due to
the obstruction of city blocks. If two queues located at two
streets and they are both in the communication range of a
vehicle in the junction area, packets from these two queues
will collide if they transmit their packets at same time. The
collisions lead to the performance degradation in city
scenario. Nonetheless, the proposed protocol still shows
performance advantages in both scenarios.
4. CONCLUSIONS
In this paper, a novel self-sorting MAC protocol is proposed
taking advantage of the characteristic of high-density
scenarios to improve the performance of high-density
VANET, since the main challenge of VANET is to meet the
communication requirements in dense situations. Vehicles
form a queue by self-sorting like a “count off” process, and
the queue reaching the specified length has the right to
accessthe channel. The control messagesfor self-sortingare
designed to be collision tolerable to reduce the collisions of
data messages. The protocol changesthe unorderedrandom
accessinto an ordered way. The variance of completiontime
of the self-sorting process introduces the time sequence
naturally when queues compete for channel access. The
analysis and simulation results evaluate the performance of
the protocol in terms of delay and PDR. In addition, the
overhead is less than the typical existing slotted MAC
protocols, which have fixed structureandrequirecontinuous
transmission of allocation information. Our future workwill
focuson adapting the protocol in variousscenariosincluding
more dynamic configurations optimization and support for
heterogeneous requirements on delay and PDR.
0.4
0.5
0.6
0.7
0.8
0.9
1
Pa
ck
et
De
liv
er
y
Ra
tio
Highway scenario
City senario
0.1 0.2 0.3 0.4 0.5 0.6 0.7
0
5
10
15
20
25
30
35
Vehicle Density (vehicles/m)
De
la
y
(m
s)
Highway scenario
City senario

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